1. Field of the Invention
This invention relates to a circuit to sequentially turn on and off a series connected string of electronic devices with a minimum switching delay while limiting the voltage drop across each device.
2. Description of Related Art
The ability to operate insulated gate bipolar transistors (IGBTs) at high voltage, and thereby benefit from their high frequency capability, is limited by their specified voltage ratings. Previous art methods for operating IGBTs in series, where the voltage across the IGBTs is higher than the maximum voltage rating of each IGBT, include the use of a voltage clamp circuit across each IGBT, and a turn-on and turn-off sequence circuit that forces the system voltage to be applied across the IGBTs in a controlled order.
In operation, as the voltage across each IGBT increases, the corresponding voltage clamp circuit clamps the voltage to a maximum value. The process of the prior art uses a short, fixed delay to ensure that each IGBT is turned on or off before the next device in the series string is operated. This delay compensates for variations in the turn-on time of each IGBT but must be longer than the slowest IGBT turn-on time expected. This turn-on and turn-off delay method requires that the ratings of the voltage clamp circuit be larger than necessary and extends the total operating time needed to operate all the IGBTs in the series string.
When solid state switching devices, such as IGBTs, transistors or SCRs (silicon controlled rectifiers), are operated in series, they do not all turn on or turn off at the same time. Unless provisions are made for limiting the voltage drop across each device, the first device to turn off must support the entire system voltage until the other devices turn off. If, during this period, the system voltage is greater than the device voltage rating, the device will fail. The same holds true when the devices are turned on. The last device to be turned on will be subjected to the full system voltage and, if it is greater than the device rating, it will fail.